1. Field of the Invention
This invention relates to an optical recording medium of phase change type.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording medium is phase change optical recording medium wherein a laser beam is directed to the recording layer to change its crystalline state whereupon a change in reflectance by the crystallographic change is detected for reproducing the information.
Phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit used for their operation is simple.
When information is recorded on the optical recording medium of phase change type, the laser beam applied is of the power (recording power) such that the recording layer is heated to a temperature higher than its melting point. In the region where the recording power is applied, the recording layer melts and the molten region is thereafter quenched to form an amorphous record mark. When the record mark is erased, the laser beam applied is of the power (erasing power) such that the recording layer is heated to a temperature higher than the crystallization temperature and lower than the melting temperature. The record mark irradiated with the laser beam of erasing power is heated to a temperature higher than the crystallization temperature and then allowed to slowly cool to recover its crystalline state. Accordingly, in the optical recording media of phase change type, the medium can be overwritten by modulating intensity of a single laser beam.
In recent years, special attention is being given to rewritable digital video discs (DVD-RAM), and a recording film of phase change type is employed in such discs. In DVD-RAM Ver. 1.0, data of 2.6 GB is recorded on one side of the disc having a diameter of 120 mm. In this case, the recording is conducted at a recording wavelength of 0.65 .mu.m, a numerical aperture NA of the optical pick up of 0.60, and a track pitch of 0.74 .mu.m and with the recording system of land/groove recording and modulation system of 8-16 modulation, and the minimum mark length is 0.62 .mu.m. Data transfer rate of 11.06 Mbps is attained by adopting the relative linear velocity of the disc in relation to the optical pick up of 6 m/s.
Jpn. J. Appl. Phys. vol.37 (1998) pp. 2104-2110 describes ZnS--SiO.sub.2 (100 nm)/interface layer (5 nm)/Ge.sub.2 Sb.sub.2 Te.sub.5 (20 nm)/ZnS--SiO.sub.2 (20 nm)/Al-alloy (150 nm) as a film structure by taking the DVD-RAM specification into consideration. The number in the brackets is thickness.
In the optical recording medium of phase change type, recording at a higher recording density and a higher linear velocity is realized by improvements either on the side of the media drive system or on the side of the medium itself. Improvements of the drive system include use of the recording laser beam having a shorter wavelength, increase in NA of the optical pick up, and the like, and improvements of the medium include narrowing of the track pitch, shortening of the minimum record mark, and the like. Of the improvement on the side of the medium, the narrowing of the track pitch is likely to result in an increased cross erase wherein the record marks on the adjacent track are erased, an increased cross talk wherein the record marks on the adjacent track are read out, and a reduced C/N. In particular, when the track pitch is less than 0.7 .mu.m, cross erase rapidly increases to invite serious increase in jitter.
In consideration of such situation, the inventors of the present invention actually conducted the recording using the film structure described in Jpn. J. Appl. Phys. vol.37 (1998) pp. 2104-2110 at a recording wavelength of 650 nm and an NA of 0.60 and using the 8-16 modulation system and the land/groove recording as in the case of DVD-RAM, and at a minimum mark length (0.42 .mu.m) and a track pitch (0.60 .mu.m) which were shorter than those of DVD-RAM. The jitter properties were unfavorable.
Japanese Patent Application Kokai No. (JP-A) 338064/1994 describes a method for improving the cross erase. In the information recording medium of JP-A 338064/1994, a recording film is provided and its reflectance is locally changed by laser beam irradiation. Grooves are preliminarily formed in the medium, and the land between two adjacent grooves is used for the information recording track. The groove depth corresponds to an optical length of .lambda./7 to .lambda./5 when the information reproducing laser beam has a wavelength of .lambda., and the groove width is substantially equal to the interval between two adjacent grooves. The beam diameter R (corresponding to the diameter of the circle at which the light intensity is 1/e.sup.2 of the light intensity at the center) of the information recording laser beam on the recording film and the groove width Wg fulfills the relation: EQU 0.34.ltoreq.Wg/R.ltoreq.1.0
There is described in JP-A 338064/1994 that the medium disclosed therein does not suffer from deterioration in quality of the reproduced signals due to partial erasure of the record marks on the adjacent information track, namely, due to the cross erase.
The track width employed in the Example of JP-A 338064/1994 is 0.7 .mu.m. JP-A 338064/1994 does not refer to the suppression of the cross erase at a track pitch smaller than 0.7 .mu.m. In the experiments conducted by the inventors of the present invention wherein experimental conditions were the same as those of JP-A 338064/1994 except for the track pitch, jitter increase due to the cross erase was observed at a track pitch of 0.6 .mu.m.
In addition, there is reported in IEEE Trans.on Magn. vol.34, No.2 (1998) p.337 that jitter increase by the cross erase takes place even when the groove depth is .lambda./6 at the wavelength .lambda. of 680 nm and the NA of 0.6, namely, at the laser beam diameter R of 929 nm, the groove width Wg of 0.65 .mu.m, and the track pitch=0.65 .mu.m, namely, at Wg/R of 0.70.